Large format multi-color sequentially illuminated LED displays (including full color LED video screens) have become available in recent years and are now in common use. LED displays typically include numerous individual LED panels providing image resolution determined by the distance between adjacent pixels or “pixel pitch.” Conventional LED displays include “RGB” three-color displays with arrayed red, green and blue emitters, and “RG” two-color displays include arrayed red and green emitters. Other colors and combinations of colors may be used.
Outdoor displays intended for viewing from great distances typically have relatively large pixel pitches and usually include discrete LED arrays. A LED array useable with an outdoor display may include a cluster of red, green, and blue LEDs that may be independently operated to form what appears to be viewer to be a full color pixel. Indoor displays may require shorter pixel pitches (e.g., 3 mm or less) and typically include panels with red, green, and blue LEDs mounted on a single electronic device attached to a driver printed circuit board (PCB) that controls the LEDs.
It is known to enclose an LED chip in a package to provide environmental and/or mechanical protection, color selection, light focusing and other functions. A LED package also includes electrical leads, contacts, and/or traces for electrically connecting the LED package to an external circuit. A conventional two-pin LED package/component 10 is illustrated in FIG. 1, including a single LED chip 12 mounted on a reflective cup 13 with a solder bond or epoxy (which may be conductive). One or more wire bonds 11 may connect the ohmic contacts of the LED chip 12 to leads 15A and/or 15B, which may be attached to or integral with the reflective cup 13. The LED package illustrated in FIG. 1 may include a vertically oriented LED chip 12 with a conductive growth substrate (p-side up in a group III-nitride LED) or conductive carrier substrate (n-side up) and one wire bond 11. In alternative implementations, a LED component may include a laterally oriented LED chip on an insulating substrate with two wire bonds. In other implementations involving use of one or more “flip” chips, the need for wire bonds may be eliminated. A transparent encapsulant material 16 may be provided in the reflective cup 13. A wavelength conversion material, such as a phosphor or other lumiphoric material, may be mixed with the encapsulant or otherwise arranged over the LED chip 12. Light emitted by the LED at a first wavelength may be absorbed by the wavelength conversion material, which may responsively emit light at a second wavelength. The assembly can be further covered with a clear protective resin 14, which may be molded in the shape of a lens to direct or shape the light emitted from the LED chip 12 and/or wavelength conversion material.
Another conventional LED package 20 is illustrated in FIG. 2, with the package 20 being suitable for high power operations with increased thermal dissipation requirements. In the LED package 20, one or more LED chips 22 are mounted over a carrier such as a printed circuit board (PCB) carrier, substrate or submount 23, which may include ceramic material. The package 20 may include one or more LED chips 22 of any suitable spectral output (e.g., ultraviolet, blue, green, red, white (e.g., blue LED chip arranged to stimulate emissions of phosphor material) and/or other colors). A reflector 24 may be mounted on the submount 23 (e.g., with solder or epoxy) to surround the LED chip(s) 22, reflect light emitted by the LED chips 22 away from the package 20, and also provide mechanical protection to the LED chips 22. One or more wirebond connections 21 may be made between ohmic contacts on the LED chips 22 and electrical traces 25A, 25B on the submount 23. The LED chips 22 are covered with a transparent encapsulant 26, which may provide environmental and mechanical protection to the chips while also acting as a lens.
Conventional LED components or packages such as shown in FIGS. 1 and 2 may include transparent encapsulant covering LED chips and reflector cups to minimize absorption of emitted light, and thereby ensure maximum light extraction. When used in LED displays, however, the reflective cups in conventional LED packages can reflect significant amounts of ambient light (e.g., sunlight incident on a LED display), which may impair viewing of images and/or text represented on the display. A conventional way to improve contrast is to position a neutral gray filter between a reflector associated with a display device and a light output surface of the device, thereby attenuating reflected ambient light nearly twice as much as direct emitted light (since ambient light incident on the display is attenuated once following passage in an incoming direction through the filter and is attenuated again after reflection following passage in an outgoing direction through the filter, whereas direct emitted light is attenuated only once upon passage in an outgoing direction through the filter.) This conventional neutral gray filtering, however, also reduces output of the direct emitted light, thereby requiring increased power and increased thermal dissipation to operate solid state emitters in order to achieve a desired level of display brightness. The art continues to seek improved LED devices and displays with reduced reflection of incident light and with enhanced contrast while overcoming limitations associated with conventional devices.